High yield byproduct recycle process for anhydro sugar alcohols

Abstract

A process for recycling reaction byproducts in the manufacture of dianhydro sugar alcohols, such as isosorbide, from corresponding sugar alcohols.

Description

This invention concerns a process for separating and recycling reaction byproducts in the manufacture of dianhydro sugar alcohols.TECHNICAL BACKGROUND OF THE INVENTIONAnhydro sugar alcohols, in particular derivatives of mannitol, iditol, and sorbitol, are known for their therapeutic uses and uses in food. At least one of these, isosorbide, 1,4:3,6-dianhydrosorbitol, is useful as a monomer used in the manufacture of polymers and copolymers, especially polyester polymers and copolymers.Anhydro sugar alcohols are produced by dehydration of the corresponding sugar alcohols (or monoanhydro sugar alcohols) by the action of various dehydration catalysts, typically strong acid catalysts.Batch processes for the preparation of dianhydro sugar alcohols by acid dehydration are known in the art.In particular, a batch process for the formation of the dianhydro sugar alcohol isosorbide has been described as a two step process involving intramolecular dehydration of sorbitol to 1,4-sorbitan (1,4-mo...

AI Technical Summary

Benefits of technology

The present invention provides a process for separating and recycling reaction byproducts and unreacted sugar alcohols in the manufacture of dianhydro sugar alcohols. This increases the overall yield of the desired product and reduces the loss of byproducts. The process involves diluting a mixture of monomeric, dimeric, and polymeric sugar alcohols with water, precipitating most of the polymeric sugar alcohols, and separating the soluble monomeric and dimeric sugar alcohols from the precipitate. This can be done in a batch, semi-batch, or continuous mode.

Problems solved by technology

In the above processes, separation of dianhydro sugar alcohols from the reaction mass is not complete, and substantial quantities of product are lost with the higher boiling byproducts.

Increasing the vaporization temperature may increase product recovery, but this adversely affects product quality, as more of the byproducts and color-forming impurities are vaporized along with the desired product.

In the case of chromatographic separation, substantially larger equipment is required to achieve higher recovery, which may not be economical.

The dehydration reactions are, however, accompanied by several side reactions that lead to generation of unwanted byproducts.

Recovery of the desired dianhydrosugar alcohol from the reaction mass is often difficult due to low volatility.

The solubility properties of isosorbide are similar to those of the sorbitan isomers, making it difficult to develop an economically effective separation by solvent extraction.

Product recovery, however, is far from complete, and a substantial quantity of the product, 20 to 30%, may be lost as unrecovered with the byproducts, particularly when recovering a product of good purity.

Recovery can be improved by subjecting the byproduct stream to higher temperatures and / or still lower pressures, but that is at the expense of higher degradation and poorer quality product.

Another practical limitation is that above 170.degree. C., the reaction mass starts to char at a rapid rate.

Addition of large quantities of water, however, is not necessary and not desirable as it increases the water load on the equipment.

This causes their rate of consumption to form polymer to increase.

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